The invention relates generally to the field of optical communications and optical signal processing. In particular, the invention relates to apparatus and methods for all-optical bit phase sensing and clock recovery.
High-speed time division multiplexed (TDM) communication systems require high-speed clock recovery or clock synchronization. Multiple-user local area and metropolitan area TDM networks require high-speed clock recovery at each user access node. Typically, this clock recovery will involve locking a local clock to an incoming data or clock stream. Electrooptical and all-optical clock recovery and clock synchronization is advantageous because it has the potential for achieving higher speeds than all-electrical clock recovery and clock synchronization.
Several optical clock recovery and clock synchronization techniques have been demonstrated that utilize injection-locking of diodes, fibers, and lasers. Also, several high-speed optical clock recovery techniques have been demonstrated that utilize electrooptical phase lock loops (PLL) with bit phase sensors. An electrooptical phase lock loop has been demonstrated that utilizes nonlinear cross-correlation of two pulse streams to sense bit phase. Another electrooptical phase lock loop has been demonstrated that utilizes four wave mixing in a semiconductor to sense bit phase. These clock recovery techniques, however, have limited scalability in data and clock rates.
Commercially practical 100-Gb/s TDM communication systems require reliable, inexpensive, clock recovery techniques with sub-picosecond accuracy and a wide range of scalability in data and clock rates. Furthermore, it is desirable for the clock recovery technique to perform optical processing functions, such as multiplexing, demultiplexing, and Boolean logic functions, simultaneously with clock recovery in a single optical switch.
It is a principal object of this invention to provide an all-optical and an electrooptical bit phase sensor with at least sub-picosecond accuracy. It is another object of this invention to provide an electrooptical and an all-optical phase lock loop that utilizes these bit phase sensors. Other objects are to provide optical processors and optical networks that utilize these bit phase sensors.
A principle discovery is that nonlinear optical switches can be utilized to recover a clock signal with sub-picosecond accuracy. Another principle discovery is that a nonlinear optical switch can be utilized to perform simultaneous optical processing and clock recovery. Another principle discovery is that an all-optical phase lock-loop can be implemented using the optical output from an optical switch and an intensity dependent delay line. Another principle discovery is that nonlinear absorption in optical fibers and semiconductors can be utilized to recover clock signals with sub-picosecond accuracy.
Accordingly, the present invention features an all-optical bit phase sensor having a first optical beam input. A splitter, which is optically coupled to the first optical beam input, separates an input optical beam into a first and a second optical beam that propagates along a first and a second optical path, respectively. A nonlinear material, that forms an intensity dependent phase or transmission change, is positioned in the first optical path. The nonlinear material may also be disposed in the second optical path. The nonlinear material may be an optical fiber or a semiconductor amplifier.
A control optical beam input couples a control optical beam into the first optical path. The control beam causes nonlinear or transmission index changes in the nonlinear material. The input optical beam and the control beam may have substantially the same group velocities and thus may have substantially zero dispersive walk through. A recombiner recombines the first and the second optical beams into an output beam. The intensity of the output beam is proportional to the relative phase between the input optical beam and the control beam. A beam removal element may be positioned in the optical path to remove the control beam from the output beam. The beam removal element may comprise a filter, polarizer, or spatial multiplexer.
The present invention also features an all-optical bit phase sensor having a first optical beam input for accepting a first optical beam into an optical path. An optical differential delay element is disposed in the optical path which forms a second optical beam in the optical path by delaying a portion of the first optical beam in time. A nonlinear material is positioned in the optical path. The nonlinear material forms an intensity dependent phase or transmission change. The nonlinear material may be an optical fiber or a semiconductor amplifier.
A second input introduces a control beam into the optical path. The control beam causes nonlinear index or transmission changes in the nonlinear material. The control beam and the second optical beam may be pulse streams that are timed to overlap in the nonlinear material. The first optical beam and the control beam may have substantially the same group velocities and thus may have substantially zero dispersive walk through. A recombiner recombines the first and the second optical beams into an output beam. The intensity of the output beam is proportional to the relative phase between the first optical beam and the control beam. A beam removal element may be positioned in the optical path to remove the control beam from the output beam. The beam removal element may comprise a filter, polarizer, or spatial multiplexer.
The present invention also features a method of all-optical bit phase sensing. The method includes splitting an input optical beam into a first and a second optical beam that propagates along a first and a second optical path respectively. A nonlinear material is positioned in the first optical path. The control optical beam is coupled into the first optical path causing nonlinear index or transmission changes in the nonlinear material. The first and second optical beams are recombined into an output beam. The intensity of the output beam is proportional to the relative phase between the input optical beam and the control beam.
The present invention also features a second method of all-optical bit phase sensing. The method includes introducing a first optical beam into an optical path. A second optical beam is formed in the optical path by delaying a portion of the first optical beam in time. A nonlinear material is positioned in the optical path which has an intensity dependent phase or transmission change. A control beam is introduced into the optical path which causes nonlinear index or transmission changes in the nonlinear material. The first and the second optical beams are recombined into an output beam. The intensity of the output beam is proportional to the relative phase between the first optical beam and the control beam.
The present invention also features an electrooptic phase lock loop having a nonlinear interferometer. The nonlinear interferometer may comprise a Mach-Zehnder interferometer, a Sagnac interferometer, a Michelson interferometer, or a single arm interferometer. The nonlinear interferometer has a first optical beam input, a control optical beam input, and an optical beam output. An output optical beam of the interferometer has an intensity proportional to a phase difference between an input intensity modulated data stream input to the first optical beam input and a control clock stream input to the control optical beam input.
A feedback control network has an optical input optically coupled to the optical beam output of the interferometer and an electrical output. The electrical output of the feedback control network generates a signal in response to the intensity of the output optical beam of the interferometer. An optical clock stream generator includes an electrical input electrically coupled to the electrical output of the feedback control network and an optical output optically coupled to the control optical beam input of the nonlinear interferometer. The optical output of the optical clock stream generator produces an optical beam having a frequency that is proportional to the electrical output of the feedback control network. When the phase lock loop is closed, the output of the optical clock stream generator tracks the phase of the input intensity modulated data stream.
The feedback control network may include a detector, a differential amplifier, and a voltage controlled oscillator. The detector is optically coupled to the output optical beam of the nonlinear interferometer. The detector generates an output electrical signal proportional to the intensity of the output optical beam of the nonlinear interferometer. The differential amplifier has a first input electrically coupled to the output electrical signal of the detector, a second input electrically coupled to a bias voltage supply, and an output. The voltage controlled oscillator has an input electrically coupled to the output of the differential amplifier and an output electrically coupled to the electrical input of the optical clock stream generator. The output of the voltage controlled oscillator produces a signal having a frequency proportional to the magnitude of the output optical beam of the nonlinear interferometer.
The present invention also features an all-optical phase lock loop having a nonlinear interferometer. The nonlinear interferometer may comprises a Mach-Zehnder interferometer, a Sagnac interferometer, a Michelson interferometer, or a single arm interferometer. The nonlinear interferometer has a first optical beam input, a control optical beam input, and an optical beam output. An output optical beam of the interferometer has an intensity proportional to a phase difference between an input intensity modulated data stream input to the first optical beam input and a control clock stream input to the control optical beam input.
A feedback control network has an optical input optically coupled to the optical beam output of the interferometer and an optical output. The optical output of the feedback control network generates an optical beam in response to the intensity of the output optical beam of the interferometer. An optical clock stream generator includes an optical input optically coupled to the optical output of the feedback control network and an optical output optically coupled to the control optical beam input of the nonlinear interferometer. The optical output of the optical clock stream generator produces an optical beam having a frequency that is proportional to the optical output of the feedback control network. When the phase lock loop is closed, the output of the optical clock stream generator tracks the phase of the input intensity modulated data stream.
The present invention also features an optical processor that performs simultaneous clock recovery and processing functions. The optical processor includes a nonlinear interferometer that may comprise a Mach-Zehnder interferometer, a Sagnac interferometer, a Michelson interferometer, or a single arm interferometer. A first input of the nonlinear interferometer accepts an input optical beam and a second input accepts a control optical beam.
A first output of the interferometer generates a first output optical beam having an intensity that is functionally related to the input optical beam and the control optical beam. A second output of the interferometer generates a second optical beam that is also functionally related to the input optical beam and the control optical beam. The second output of the interferometer may be a portion of the first output of the interferometer. The second output of the interferometer may generate a demultiplex function or any Boolean logic function, such as an AND function, a NOT function, a XOR function, a OR function, a NOR function, or an INVERT function.
The optical processor also includes a feedback control network having an optical input optically coupled to the first output of the interferometer and an electrical output. The electrical output produces a signal having a magnitude functionally related to the intensity of the output of the interferometer. The optical processor also includes an optical beam generator having an electrical input electrically coupled to the electrical output of the feedback control network and an optical output optically coupled to the first or the second input of the nonlinear interferometer. The optical output of the optical beam generator produces an optical clock stream having a frequency that is proportional to the electrical output of the feedback control network.
Alternatively, the optical processor includes a feedback control network having an optical input optically coupled to the first output of the interferometer and an optical output. The optical output produces a signal having a magnitude functionally related to the intensity of the output of the interferometer. The optical beam generator includes an optical input optically coupled to the optical output of the feedback control network and an optical output optically coupled to the first or the second input of the nonlinear interferometer.
The present invention also includes an optical network including at least one network optical fiber and a clock recovery system that is optically coupled to at least one network optical fiber. The clock recovery system includes a nonlinear interferometer having a first input optically coupled to at least one network optical fiber, a second input, and an output. The clock recovery system includes a nonlinear interferometer that may comprise a Mach-Zehnder interferometer, a Sagnac interferometer, a Michelson interferometer, or a single arm interferometer.
The clock recovery system also includes an optical pulse generator having an electrical input and an optical output optically coupled to the first or the second input of the nonlinear interferometer. The clock recovery system also includes a feedback control network having an input optically coupled to the output of the nonlinear interferometer and having an output electrically coupled to the electrical input of the optical pulse generator. The frequency of an optical beam produced by the optical pulse generator is proportional to the output of the feedback control network.
Alternatively, the clock recovery system includes an optical pulse generator having an optical input and an optical output optically coupled to the first or the second input of the nonlinear interferometer. The clock recovery system includes a feedback control network having an input optically coupled to the output of the nonlinear interferometer and having an output optically coupled to the optical input of the optical pulse generator.
The feedback control network may include a detector, an amplifier, and a voltage-controlled oscillator. The detector is optically coupled to the output of the nonlinear interferometer and electrically connected to the amplifier. The amplifier is electrically connected to the voltage controlled oscillator. The voltage controlled oscillator is electrically connected to the optical pulse generator.
The present invention also features an all-optical bit phase sensor comprising an optical fiber having a core. The optical fiber may be an erbium doped fiber. The fiber includes a first and a second input that introduces a first and a second overlapping pulse stream, respectively, into the core. The first input and the second input may be optically coupled to an end of the optical fiber. The all-optical bit phase sensor also includes a detector positioned perpendicular to a longitudinal direction of the core and optically coupled to the core.
The present invention also features an optical network comprising at least one network optical fiber and a clock recovery system that is optically coupled to the network optical fiber. The clock recovery system includes an optical fiber having a core, a first input coupled to at least one network optical fiber, and a second input. The optical fiber having the core may be a erbium doped fiber.
The first and second inputs introduce a first and a second overlapping optical pulse stream into the core. The clock recovery system also includes an optical pulse generator optically coupled to the second input that generates the second optical pulse stream. The clock recovery system also includes a detector that is optically coupled to the core of the network optical fiber. In addition, the clock recovery system includes a feedback control network having an input optically coupled to the output of the detector and having an output electrically coupled to the optical pulse generator.
The present invention also features a third method of all-optical bit phase sensing. The method includes providing an optical fiber having a core, a first input, and a second input. A first and a second overlapping pulse stream are introduced into the first and the second input of the optical fiber, respectively. Light emitted through the core is detected.
The present invention also features an electrooptical bit phase sensor comprising a semiconductor diode having an active layer. The diode includes a first and a second input that introduce a first and a second overlapping pulse stream, respectively, into the active layer. The bit phase sensor also includes a voltage detector that is electrically coupled to the active layer. The detector measures an output electrical signal having a voltage proportional to the phase difference of the first and the second overlapping pulse stream.
The present invention also features a method of electrooptical bit phase sensing. The method includes providing a semiconductor diode having an active layer, a first input, and a second input. A first and a second overlapping pulse stream are introduced into the first and the second input of the active layer, respectively. A detector measures an output electrical signal that has a voltage proportional to the first and a second overlapping pulse stream.
The present invention also features an optical network comprising at least one network optical fiber and a clock recovery system that is optically coupled to at least one network optical fiber. The clock recovery system includes a semiconductor diode having an active layer. The diode also includes a first and a second input that introduces a first and a second overlapping pulse stream, respectively into the active layer.
The clock recovery system also includes an optical pulse generator optically coupled to the second input that generates the second optical pulse stream. The clock recovery system also includes a voltage detector having an electrical input electrically coupled to the active layer. In addition, the clock recovery system includes a feedback control network having an input electrically coupled to the output of the detector and having an output electrically coupled to the optical pulse generator.